Associated with increases in office automation based around computer networks over the past ten years, there has developed an increasing desire to reduce, and ultimately to eliminate, data cable connections between devices such as main frame computers, terminals, personal computers, printers and the like. This has resulted in the development of so-called "wireless local area networks" (wireless LAN's), which utilise bi-directional radio frequency (RF) communications between devices arranged within such a system. Such systems present essentially three basic problems that have hampered their development. Firstly, in view of the number of individual number of equipments generally used in such a system, the hardware cost of any RF transceiver to be associated with each item of equipment should be minimized to ensure cost effectiveness. Further, the desired data rate, which is currently approximately 2 megabits per second (Mbps), and which is anticipated to increase to approximately 10 Mbps by the year 2000 generally necessitates microwave transmission frequencies, which are typically used for point-to-point communications over substantially larger distances. The third requirement is that of transmitter power. On this last point, the transmitted power level must be sufficiently low to fall within acceptable limits of environmental radiation hazards associated with office work places, and yet have sufficient power to achieve adequate propagation.
QAM spread spectrum systems when implemented at a high level, for example 64 QAM, have the advantages of being able to provide a high spectral efficiency which allow data rates in excess of 10 Mbps to be achieved whilst meeting the processing gain requirements established by United States Regulation FCC15.247, and a similar Japanese regulation, in a bandwidth of less than 26 MHz. Also, such systems provide an improved peak/average power ratio relative to other known transmission systems.
However, such systems have disadvantages. In particular, the traditional implementation of 64 QAM demodulation with a symbol rate of around 2 Mbps is complex, and as a result, not generally suited to the size and power requirement necessary for wireless LANs. Further, the low distance between symbols in a 64 QAM constellation causes susceptibility to multi-path effects, otherwise known as frequency selective fading. Additionally, 64 QAM is highly susceptible to amplitude modulation to phase modulation (AM/PM) conversion that occurs within the power amplifier of the RF transmitter. Traditional techniques to overcome this problem include designing expensive, highly linear power amplifiers, or alternatively operating power amplifiers approximately 10 dB below the rated output. Such techniques generally do not meet the cost-requirements for wireless LAN applications.
U.S. Pat. No. 5,005,186 (Aono et al) discloses a quadrature amplitude modulation (QAM) spread spectrum demodulator which permits high data rate and relatively low cost implementation through the use of a read-only-memory (ROM) which operates for constellation decoding and to provide error correction signals. However, such an arrangement is understood to be configured for microwave point-to-point applications and is generally unsuitable to wireless LAN environments.
Accordingly, it would be advantageous to provide a radio frequency transmission system which at least ameliorates the deficiencies mentioned above and thereby permits implementation in wireless LAN applications.